Today's communications grade optical fiber of fused silica, SiO2, is manufactured according to three basic steps: 1) core preform or “start rod” fabrication, 2) core-with-cladding preform fabrication, and 3) fiber drawing. The core and cladding of a preform correspond in ratios and geometry to those of the ultimate glass fiber that is drawn from the preform.
The first step is to build up a start rod, forming it into a glass that will eventually become the fiber's core, and in some cases, also part of the fiber's cladding layer. The start rod is a glass rod made of silica, SiO2, with the portion of the start rod that comprises the core being doped with a small amount of a dopant, typically Germania, GeO2. The presence of the dopant in the core increases the refractive index of the glass material compared to the surrounding outer (cladding) layer. In the second step, a cladding layer is built up on the start rod. The result of this step is a preform having a core and a cladding, which is conventionally about 160 mm in diameter and about one meter long. The third step is fiber drawing, where the preform is heated and stretched, and typically yields about 400 km of optical fiber.
The primary raw ingredient to fabricating the glass preform is silicon tetrachloride, SiCl4, which generally comes in a liquid form. As noted above, however, SiO2, typically in the form of glass soot, is deposited on the start rod. The chemical reactions involved in the formation of the glass soot are complex, involving, SiCl4, oxygen, O2, and the fuel gas combustion products. In all of the techniques, the silica, SiO2, comprises the cladding of the preform according, generally, to the reaction:SiCl4+O2═SiO2+2Cl2.
Generally, there are four distinct technologies for fabricating core preforms. These technologies include Modified Chemical Vapor Deposition (MCVD), Outside Vapor Deposition (OVD), Vapor Axial Deposition (VAD), and Plasma Chemical Vapor Deposition (PCVD). The resulting product for all of these technologies is generally the same: a “start rod” that is generally on the order of one meter long and 20 mm in diameter. The core is generally about 5 mm in diameter.
Similarly, there are generally four technologies for performing the step of adding the cladding. These technologies include tube sleeving (conceptually paralleling MCVD), OVD soot overcladding (conceptually paralleling OVD), VAD soot overcladding (conceptually paralleling VAD), and plasma (conceptually paralleling PCVD). In this step, additional cladding layers of pure or substantially pure fused silica are deposited on the start rod to make a final preform that can be prepared for fiber drawing.
In MCVD, the step of manufacturing the start rod is performed inside of a tube. Similarly, when the cladding step is performed, a larger tube is sleeved onto and fused to the start rod. Presently, the company, Heraeus, manufactures tubes used for producing start rods and for sleeving onto and fusing with the start rods to make preforms.
In OVD, when fabricating start rods, glass is deposited onto a rotating mandrel in a “soot” deposition process. The start rod is slowly built up by first depositing the germanium-doped core, and then the pure silica layers. When the core deposition is completed, typically the mandrel is removed and then the remaining body is sintered into a start rod of glass.
In the process of OVD soot overcladding, where a cladding is deposited onto a fabricated start rod, the start rod is rotating and traversing on a lathe such that many thin layers of soot are deposited on the rod in a stream from a chemical deposition burner over a period of time. Although the SiO2 is not deposited onto the start rod as a vapor, but rather as SiO2 particles, the process is known in the art as a “chemical vapor deposition” process because the SiCl4, which reacts in the stream between the burner and the start rod to form SiO2, is input to the burner as a vapor. The porous preform that results from the OVD soot overcladding process typically is then sintered in a helium atmosphere at about 1500° C., into a solid, bubble-free glass blank. U.S. Pat. No. 4,599,098, issued to Sarkar, which is incorporated by reference as though fully set forth herein, provides further background on systems and techniques for OVD and OVD soot overcladding.
For the above-referenced technologies, typically any one of the core fabrication technologies may be combined with any one of the cladding fabrication technologies to generate a preform that may be used for drawing fiber.
In the OVD soot overcladding processes, one of the key measures of economic viability in comparison to the other available techniques is the deposition rate of the SiO2 on the workpiece. For example, some companies involved in optical fiber manufacturing opt for the most cost-effective method of performing the step of overcladding the start rod in the fiber manufacturing process. With respect to this step in the process, the choice is either to purchase the cladding tubes or to perform a deposition process to add the cladding.
In considering the different approaches, the economics often are reduced to a question of whether a particular vapor deposition system that a company is considering maximizes the deposition rate-to-cost ratio. The deposition rate may be characterized, for example, by the average grams/minute of silica soot that can be deposited on the start rod until completion (i.e., an optical fiber preform ready for sintering). Above a certain average deposition rate, performing the soot overcladding process is likely to be economically more attractive to the company than purchasing cladding tubes. Companies that manufacture systems for performing soot overcladding focus on achieving the highest possible deposition rates and being cost effective, but without compromising the quality of the preform that is produced for fiber drawing.
The factors that determine a deposition system's deposition rate are the chemical vapor delivery rate and the efficiency of chemical vapor deposition onto the workpiece. With respect to vapor delivery, key issues generally revolve around continuously and efficiently maintaining a high (e.g., greater than 200 grams/minute) delivery rate over a prolonged period (e.g., greater than 2 hours). Several methods have been described in the prior art for supplying a hydrolyzing burner with a substantially constant flow of vaporized source material entrained in a carrier gas. For example, in U.S. Pat. No. 4,314,837 issued to Blankenship (“the Blankenship reference”), a system is described that includes several enclosed reservoirs each containing liquid for the reaction product constituent. The liquids are heated to a temperature sufficient to maintain a predetermined vapor pressure within each reservoir. Metering devices are coupled to each reservoir for delivering vapors of the liquids at a controlled flow rate. The respective vapors from each reservoir are then combined before they are delivered to the burner.
With respect to enhancing the deposition efficiency of SiO2 on the workpiece to improve the effective deposition rate, studies have been performed to characterize the flow of chemical vapor from the burners to the surface of the workpiece in the reaction chamber. One reference directed to this issue is Li, Tingye, Fiber Fabrication, pp. 75-77, Optical Fiber Communications, (Academic Press, Inc. 1985). As discussed in the above reference, because of the small size of the formed glass particles, momentum does not cause an impaction of the particles onto the surface of the workpiece. The small sizes of the glass particles would tend to force them to follow the gas stream around instead of at the preform surface. Rather, the phenomenon of thermophoresis is the dominant mechanism for collection on the surface of the preform. As the hot gas stream and glass particles travel around the workpiece, a thermal gradient is established near the surface of the preform. Preferably, the thermal gradient is steep, effectively pulling the glass particles by a thermophoretic force towards the preform.
Various methods have been proposed to increase deposition efficiency based on establishing and maintaining the thermophoretic force. One method is to vary the distance between the burner and the workpiece. See H. C. Tsai, R. Greif and S. Joh, “A Study of Thermophoretic Transport In a Reacting Flow With Application To External Chemical Vapor Deposition Processes,” Int. J. Heat Mass Transfer, v. 38, pp. 1901-1910 (1995). Another set of methods, disclosed in U.S. Pat. Nos. 6,789,401, 7,451,623 and 7,451,624 issued to Dabby et al., which are incorporated herein by reference as though fully set forth herein, involves selectively translating the burners relative to the workpiece above various threshold velocities, e.g., greater than 1.4 meters per minute. Higher velocities mean that less heat is applied to any given spot on the workpiece. The workpiece is therefore kept cooler, which tends to increase the thermal gradient. Nevertheless, even applying these methods, demand for even higher deposition rates has gone unmet.
To further increase deposition rate, some have suggested providing an array of numerous soot-depositing burners. These additional burners are proposed to be positioned for example, on a burner block along the longitudinal axis of the lathe, where each burner deposits chemical soot on the workpiece. Specifically, U.S. Pat. No. 6,047,564 issued to Schaper et al., and incorporated herein by reference as though fully set forth herein, discloses a vapor deposition system in which a row of twelve equally-spaced chemical burners is mounted on a burner block, each such burner depositing soot. The burner block moves forward and backward along the longitudinal axis of the workpiece. The amplitude of the motion of the burner block generally corresponds to the distance between the burners such that each burner is deposits soot on a designated segment of the entire workpiece.
Similarly, U.S. Pat. Nos. 5,116,400 and 5,211,732 issued to Abbott et al. and incorporated herein by reference as though fully set forth herein, disclose a deposition system comprising an array of chemical burners. Like in the Schaper et al. patent, the Abbott et al. patents disclose a deposition process in which each burner in the chemical burner array deposits soot on only a portion of the usable length of the preform. The Abbott patents disclose an array of eleven burners preferably equally spaced from each other by about four inches. The burner array is oscillated through a total distance 2J, with a distance J in each direction from the burner array's center position. The Abbott et al. patents disclose that preferably the oscillation amplitude is equal to or slightly greater than the burner spacing d in order to insure uniformity of deposition. Accordingly, each burner traverses approximately 20% of the length of the preform. In discussing varying the number of burners and their spacing to improve deposition efficiency, the Abbott et al. patents disclose that for its configuration, deposition efficiency should improve as the number of burners is increased.
Such multiple burner configurations as disclosed in the Schaper et al. and Abbott et al. patents are not commercially attractive in part because anticipated improvements in deposition rate have not been realized. The close proximity of the chemical burners to each other compromises the thermophoretic effect such that the deposition efficiency significantly reduced. The close proximity of chemical burners positioned over the length of a workpiece prevents regions of the workpiece from having sufficient time to cool before another burner is delivering soot on that same region. Furthermore, the amount of heat and number of chemical streams generated in the chamber caused by having a large number of burners depositing soot compromises the desired laminar flow around the workpiece, which thereby reduces the needed thermal gradient for thermophoresis to occur. Without the optimal temperature gradient between a burner and the workpiece, thermophoresis is weakened, which reduces the deposition efficiency, and thereby, the overall deposition rate.
The costs associated with such multiple burner configurations are also prohibitive. These costs include not only the costs associated with the additional chemical burners, but the costs of the vaporizers, preheaters and other equipment needed to support them, as well as the scrubbers and other equipment needed to handle the additional wasted deposition material and heat that the burners produce. Furthermore, because these burner configurations require vast amounts of chemical to achieve acceptable deposition rates, the cost of the chemical needed to manufacture each preform is increased. These multiple-burner configurations can therefore be fairly characterized as “brute force” approaches that are unduly wasteful of material and unnecessarily expensive.
As a result, a need exists for systems and methods that offer further improvements to deposition efficiency, chemical delivery and, thereby, the overall deposition rate of chemical vapor. A need further exists for systems and methods that offer cost effective manufacturing of optical fiber preforms, including the manufacture of larger preforms in the same deposition space, and accordingly, cost-effective optical fiber. A need further exists for multiple-burner configurations in chemical vapor deposition systems and processes that maintain the necessary thermophoresis to provide higher deposition rates and efficiencies.